Although iron and ferrous alloys provide good structural mechanical properties such as strength and toughness, they are frequently deficient in their surface characteristics such as hardness and resistance to oxidative attack. Protective coatings are applied to overcome these surface deficiencies. For example, boron can be added in a diffusion coating to improve the wear resistance of carbon steels. Silicon is applied in a diffusion coating to improve corrosion and high temperature oxidation resistance of ferrous alloys.
It has been known for several decades that steels and high alloy steels can be borided by using a mixture of diborane and hydrogen at temperatures of 550.degree. to 950.degree. C. Iron, steel, nickel and cobalt surfaces can be hardened in this manner by producing a metal boride layer as thin as 5 microns, although boronized layers having thicknesses as high as 20 to 200 microns and containing FeB and Fe.sub.2 B are known. Another method of gas phase boriding of steels is by the use of BCl.sub.3 in gas mixture with hydrogen and nitrogen using similar temperatures of about 550.degree. to 950.degree. C. BCl.sub.3 and hydrogen can be used to boronize steels to produce a boronized layer having a thickness of 50 to 250 microns.
The use of silicon and boron together to form a protective oxide coating on a metal surface is described by British Patent 1,511,353 (1978). This patent describes forming a protective coating of 20 to 85 wt % silicon oxide and 80 to 15 wt % boron oxide on a metal surface by passing over the surface which has been heated between 300.degree. and 1500.degree. C. a gas mixture of silane, diborane, oxygen and an inert carrier gas, the temperature of the gas being at least 50.degree. C. below that of the surface. The oxide coatings are said to provide improved corrosion resistance, but are limited to operating temperatures below 1500.degree. C.
Nicoll, et al., Thin Solid Films, vol. 64, pages 321-326 (1979) discloses the addition of boron to silicon diffusion coating on nickel-base superalloys using chemical vapor deposition in which the chemical vapor is hydrogen containing both silicon tetrachloride and borontrichloride in a single treatment. The presence of boron is said to improve mechanical properties of the coating.
A review of the state of the art regarding boron surface treatment of metals and engineering alloys in order to increase surface hardness is given by Dearnley, et al., Surface Engineering, vol. 1, pages 203-217 (1985). Boriding is said to be unsuited for high alloy steels because FeB formation results in a thin, poorly adherent boride layer. Steels containing large quantities of silicon are said to be unsuited to boriding because of the formation of a ferrite stabilized region, adjacent the boride layer, which remains soft. Several boriding techniques are described. Packed boriding, the most favored method for safety and simplicity, involves embedding the component to be treated in a boriding powder such as B.sub.4 C. Inert diluents include silicon carbide or aluminum oxide. Paste boriding is another technique in which B.sub.4 C suspension in a binder is coated on the component. Liquid phase boriding using a salt bath, e.g. Na.sub.2 B.sub.4 O.sub.7, can be either electroless or electrolytic. Gas phase boriding can be carried out by thermal decomposition of diborane or by the reduction of boron chloride with hydrogen, optionally diluted with nitrogen to reduce FeB production. Plasma phase boriding is yet another possible technique. Multicomponent boriding is said to have been accomplished by electrolytic salt bath and paste techniques, but most interest has been focused on the pack methods. Borosiliconizing is said to be accomplished using the pack technique to boride a steel substrate and then siliconize it at 900.degree. to 1000.degree. C., resulting in the formation of FeSi in the layer which helps corrosion-fatique endurance. Chemical vapor deposition, CVD, by gas phase treatment is not suggested for boron siliconizing but is described for deposition of metal borides, e.g. WB, ZrB.sub.2 and TiB.sub.2. CVD of boron from hydrogen/boron trichloride gas mixtures is described at temperatures of about 1050.degree. to 1250.degree. C. and is said to depend on substrate temperature, supersaturation of gaseous reaction product in the gas in equilibrium with the substrate, gas flow conditions and treatment time.
Commercially, Boroloy Industries' C-1 coating system is a boron silicide diffusion coating prepared using pack cementation.
In the semiconductor industry, boron has been used to improve the formation of silicon layers on silicon wafer substrates. Eversteyn, et al., J. Electrochem., vol. 120, pages 106-110 (1973) disclose that the deposition rate of silicon films from SiH.sub.4 gas systems can be doubled by the addition of B.sub.2 H.sub.6. The polycrystalline silicon layers deposited in the presence of B.sub.2 H.sub.6 are said to have a denser structure compared to undoped growth. Nakayama, et al, J. Electrochem., vol. 133, pages 1721-1724 (1986) disclose that deposition rates of silicon on silicon wafer substrates by CVD using Si.sub.2 H.sub.6 in helium can be increased by the addition of B.sub.2 H.sub.6 to the gas system.
Improvements in the method of forming silicon diffusion coatings are disclosed by our U.S. Pat. No. 4,714,632, Cabrera, et al. (1987) which describes producing silicon diffusion coatings on a metal surface (such as iron and ferrous alloys) using silane, either alone or diluted with hydrogen. The coatings are formed at temperatures below 1000.degree. C. The metal surface is pretreated with a reducing atmosphere, such as hydrogen. Surface silicon can subsequently be oxidized to silicon dioxide to provide oxidation protection. U.S. Pat. No. 4,822,642, Cabrera, et al. (1989) describes similarly forming silicon diffusion coatings on the surfaces of nonferrous metals at temperatures below 1200.degree. C.
While silicon diffusion coating improve the resistance of iron or ferrous alloy surface to oxidation, carburization, sulfidation and corrosion, a tendency of mechanical failure has limited the coating life. For example, in oxidizing environments, the coatings tend to crack and become undermined by oxidation of the underlying metallic substrate. Boriding should further improve resistance to wear and galling, provide some corrosion resistance to the metal, and improve mechanical properties of the coating. Although pack cementation methods which have been used to borosiliconize steel are safe and simple, it is desired to obtain better control of the ratios of the diffusion elements and thereby better control over the composition of the diffusion coating.